User`s manual

535 User's Manual Chapter 7 93
Applications
for process noise. Attempting to do this will result in degraded controller
performance. The Noise Band is the distance the process deviates from
the setpoint due to noise in percentage of full scale.
Figure 7.13 shows a typical process variable response in a steady-state
situation. In this example, the process noise is within a band of about 0.5%
of full scale.
A noise band that is too small will result in tuning parameter values based
on noise rather than the effects of load (and setpoint) changes. If the noise
band is set too small, then Adaptive Tune will attempt to retune the controller
too often. This may result in the controller tuning cycling between desirable
system tuning and overly sluggish tuning. While the result may be better
than that achieved with a non-adaptive controller, this frequent retuning is
not desirable.
If the noise band is set too large, the process variable will remain within the
noise band, and the controller will not retune itself. With too large a noise
band, important disturbances will be ignored, and the controller will be
indifferent to sluggish and oscillatory behavior.
Noise band settings are generally between 0.1% and 1.0%, with most
common settings of 0.2% or 0.3%. Figure 7.14 shows the conversion of
peak-to-peak noise to an appropriate noise band for each T/C type & RTD.
3. Set RESP. TIME.
The response time is the most critical value in Adaptive Tuning. Response
time represents the time lag from a change in valve position (controller
output) to a specific amount of change in process variable. Specifically,
Response Time is equal to the Deadtime of the process plus one Time
Constant. The Deadtime is the time between initiation of an input change
and the start of an observable response in the process variable. The Time
Constant is the interval of time between the start of that observable response
and the point where the process variable reaches 63% of its final value. (See
Figure 7.15).
Example
After a stimulus (e.g., valve movement), if it takes 300 seconds for a process
to reach 63% of its new (expected) value, the response time is 300 seconds.
If the response time is set too short, the process will be unstable and cycle
Figure 7.14
Noise Band Values for
Temperature Inputs
DT τ
RT
63% of Final PV
PV
Time ➜
Final PV
Control Output
DT = Dead Time
τ = Time Constant
RT = Response Time
Figure 7.15
Deadtime and Time Constant
B E J K N R/S T W/WS PLATINEL RTD 0.1°RTD
0
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
1
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
2
0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.2
3
0.1 0.2 0.2 0.1 0.1 0.1 0.3 0.1 0.2 0.2 0.3
4
0.1 0.2 0.2 0.1 0.1 0.1 0.4 0.1 0.2 0.2 0.5
5
0.2 0.2 0.2 0.2 0.2 0.2 0.5 0.1 0.2 0.3 0.6
6
0.2 0.3 0.3 0.2 0.2 0.2 0.6 0.1 0.3 0.3 0.7
7
0.2 0.3 0.3 0.2 0.3 0.2 0.6 0.2 0.3 0.4 0.8
8
0.2 0.4 0.4 0.3 0.3 0.3 0.7 0.2 0.4 0.4 0.9
9
0.3 0.4 0.4 0.3 0.3 0.3 0.8 0.2 0.4 0.5 1.0
10
0.3 0.4 0.4 0.3 0.4 0.3 0.9 0.2 0.4 0.5 1.1
INPUT TYPE
Peak to Peak Noise °F